Watermarking of information signals is a technique for the transmission of additional data along with the signal. For instance, watermarking techniques can be used to embed copy write and copy control information into video signals.
U.S. Pat. No. 5,809,139 describes a digital watermarking method and apparatus for the watermarking of a digital video signal in a compressed form. To allow the system to be used with transmission channels having strict bit-rate constraints, the system also includes bit-stream control to prevent an increase in the bit-rate of the video signal. For each transform coefficient of the video signal, it is determined whether more bits will be necessary to encode a watermarked version of the coefficient than an un-watermarked coefficient. If more bits are required to transmit a watermark coefficient, then the un-watermarked coefficient is output. Such a method can be used to watermark a MPEG2 video Elementary Stream (E.S.).
Equally, a method described in WO 02/060182, by which a watermark is embedded by selectively discarding the smallest quantized DCT (Distribute Cosine Transform) coefficients, can be used to provide a watermarked signal that is either the same bit-rate as, or a lower bit-rate than, the original signal. This lower bit-rate can be raised back to the original bit-rate by accumulating a relatively large amount of data sufficient to form one or more packets, and stuffing the packet(s) with zero bits.
However, such methods are not appropriate for watermarking information bit-streams that are defined on the system level, for instance a Transport Stream (T.S.), as for instance used in Digital Video Broadcasting (DVB), or a Program Stream (P.S.), as for instance used in Digital Versatile Disk (DVD).
In T.S. and P.S., bit-rate control is more difficult because the elementary stream information is interrupted at random places by other data, such as other video stream(s), audio steam(s), system data, and timing data. In order to prevent distortion of the resulting video signal, it is important that the bit-rate control is such that the position of the start code in the compressed video stream does not change upon the watermarling applied. Equivalently, it is desirable that the number of bits between start codes does not change due to the watermark being applied to the signal. For instance, in MPEG a change of the number of bits between start codes would result in system timing errors and/or result in buffer over or under flow upon decoding of the bit-stream.
It is an aim of embodiments of the present invention to provide methods and apparatus suitable for applying a watermark to an information bit-stream without changing the bit-rate of the information signal.
It is a further aim of embodiments of the present invention to provide methods and apparatus for watermarking a Program Stream or a Transport Stream such that the position of start codes in the bit-stream does not change.